Modern imaging techniques in medicine such as, for example, computed tomography, ultrasound and magnetic resonance tomography, facilitate for doctors the noninvasive diagnosis relating to an examination region of a patient. The various imaging techniques are suitable with different success for qualitatively high value scans depending on the tissue to be examined. Because of its good soft part contrast, which is well known, the magnetic resonance technique is highly suitable for soft part display. It renders it possible, inter alia, to scan the same tissue with different contrasts, and thus to highlight various regions of the examination zone differently, depending on requirement.
The magnetic resonance technique (the abbreviation MR stands below for magnetic resonance) is a known technique with the aid of which images of the interior of an examination object can be produced. In simple terms, to this end the examination object is positioned in an MR unit in a comparatively strong static, homogeneous basic magnetic field (field strengths of 0.2 tesla to 7 tesla and more) such that the nuclear spins thereof are oriented along the basic magnetic field. To excite nuclear spin resonances, radiofrequency excitation pulses are irradiated into the examination object, the excited nuclear spin resonances are measured, and MR images are reconstructed on the basis of them. Rapidly switched magnetic gradient fields are superposed on the basic magnetic field for local coding of the measured data. The recorded measured data are digitized and stored as complex numerical values in a k space matrix. An associated MR image can be reconstructed from the k space matrix occupied with values by means of a multidimensional Fourier transformation.
A diagnosis frequently requires ability to distinguish healthy tissue from diseased tissue. A specific technique of visualizing diseased—for example acutely necrotic or scarred—tissue is late enhancement or delayed enhancement. In this case, use is made of the fact that in the course of time an administered contrast agent containing gadolinium, magnesium or iron, for example, is enriched later in diseased tissue than in healthy tissue, and this is visualized by means of suitable MR sequences. Healthy areas from which the contrast agent has already been washed away again after a waiting time therefore emit an only weak signal—and therefore appear dark, whereas enriched areas develop strong signals and are therefore displayed bright. The maximum contrast is typically achieved 10 to 15 minutes after administration of contrast agent. This technique therefore offers a good contrast between healthy, nonenriched tissue areas, and diseased, enriched tissue areas.
EP 0 994 352 A2 discloses an application of a late enhancement for examining microcardial. However, the contrast between diseased tissue and healthy tissue is certainly good, depending on the sequences used, but the frequently additionally desired contrast between diseased tissue and blood is generally not sufficient to enable a reliable distinction. For example, in the case of inner layer damage to the heart after a heart attack, this damage can frequently not be delimited from the adjoining blood volume. Again, the dosage of the contrast agent, the waiting time and the washout rate influence the image quality and can complicate a diagnosis.
In order to circumvent the problems named, it is possible to intercompare a number of image data obtained in various ways. A method for simultaneously recording two late enhancements is disclosed in an article by P. Kellman et al. (Multicontrast Delayed Enhancement Provides Improved Contract Between Myocardial Infarction and Blood Pool, Journal of Magnetic Resonance Imaging 22: 605-613, 2005). One scan is recorded with a T1 weighted contrast, the other with a T2 weighted contrast. By comparing the two scans, it is intended here to be able to distinguish infarcated myocardial tissue more effectively from blood. This comparison can be performed, inter alia, by taking the quotient of the two scans. However, in this context difficulties are mentioned with this approach since, instead of a desired increase in the contrast between infarcated myocardial and blood, the exact opposite, a reduction in this contrast, has also been obtained.